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NASA 

Technical Memorandum 82156 



Fourier Spectroscopy 
on Planetary Missions 
Including Voyager 



Rudolf A. Hanel 



{NAl>il--i'M-bJ15o) i-OUIUHi SPiiCI&OSCUPX Oh 
BLA^mhUY MISSIONS INCLUDING VOIAGER IUASA\ 
30 p dC AUJA11. Aul CSct Q3B 



JUNE 1981 



N81-JUU67 



Unclas 
G3/91 JJ27y 






National Aeronautics and b] "" V^-.V^'^ ^t^ ^j 

Space Administration tei t.v^Jlt-t?^^^. --^1 

Goddard Space Flight Center \p, "^^ig^^ J;/ 

Qreenbelt, Maryland 20771 -•^■'- ^^ 



I 






Mfr^f^j.^:^ 



Fourier spectroscopy on planetary ninaluns Including Voyager 

Rudolf A. Hanel 

Laboratory for Extraterrestrial Physics, NASA/Goddard Space Flight Center 

Greenbelt, Maryland 20771 



Abstract 

In the last dozen years spaceborne Fourier Transform Spectroaeters have 
obtained Infrared emission spectra of Earth, Mars, Jupiter, Saturn and Titan 
as well as of the Galilean and other Saturnlan satellites and Saturn's rings. 
Intercomparisons of the properties of planetary atmospheres and of the 
characteristics of solid surfaces are now feasible. The principles of 
remotely sensing the environment on a planetary body are discussed. Special 
consideration is given to the most recent results obtained by the Voyager 
infrared investigation on the Saturn system. 

Introduction 

For more than a decade spaceborne Fourier transform spectrometers have 
explored the Earth and other objects In the solar system. Michelson 
interferometers flown on Nimbus j in 1969^*'' and on Nimbus U In 1970^*'^'^ 
recorded more than a ■illion spectra of the Earth's atmosphere. More 
recently, a Russian meteorological satellite observed Earth with a similar 
type of inter feroneter constructed In East Germany. An advanced version of 
the Nimbus instrument was flown on the Mariner 9 orbiter in 1971/72, allowing 
investigation of the infrared spectrum of Mars.'' '' In 1979 the two Voyager 
spacecraft transmitted nmerous spectra of Jupiter, Amalthea, and the Galilean 
satellites. ' * In November 1980, Voyager l passed through the Saturnlan 



system and observed the planet, its rings. Titan and several other 
satellites. ^ Under present plans. Voyager 2 will fly by Saturn Ir 
1981, pass Uranus in 1986, and arrive at Neptune in 1989. 

1 



These space ventures have demonstrated the power of remote sensing with 
Fourier transform spectrometers. The wide spectral range at moderately high 
spectral resolution, the precise wavenumber and radiometric calibrations, and 
the reliability achieved by these instruments, have permitted scientific 
investigations which would otherwise have been impossible. 

Weight limitations and the long flight durations prohibited the use of 
highly sensitive, cryogenically cooled detectors on these missions. 
Thermistor bolometers and thermopiles were used at ambient instrument 
temperatures. Even the bes»' of these thermal detectors is far from being 
background-noise limited; tlv»refore, the multiplex advantage of the Michelson 
interferometer was fully realised. The second important property of Michelson 
interferometers, the large throur.hput or area-tiraes-solid-angle, was also used 
advantageously. The half-meter telescope of the Voyager IRIS was designed to 
match the All of the interferometer and feed it with sjJiniraum losses. The third 
advantage of the Michelson interferometer over conventional techniques is its 
wavenuraber precision. The spectra, shown in Fig. 1, of five solar system 
bodies with substantial atmospheres were recorded years apart by different 
instruments on different spacecraft, but the corresponding spectral features 
of COp and HpO on Earth and Mars, for example, and of CH^ on Earth, the giant 
planets and Titan, fall precisely at the correct wavenumbers. This precision 
is of great help in identifying unknown constituents. Finally, the results 
from all missions have shown that interferometers can be calibrated in an 
absolute sense at least as well as conventional radiometers. The usual method 
of calibration of an infrared instrument in space is to expose the field of 
view to blackbodies of different temperature. In the case of the 
interferometer, each narrow spectral interval is calibrated independently. In 
contrast, the calibration of a radiometer applies to the entire passband of 
the instrument. A wavenumber dependent change of responsivity within this 
band could pass unrecognized in a radiometer but would be detected in the 
interferometer calibration. A method was developed on Voyager which allowed 
calibration of the whole system including the telescope, simply by viewing 
deep space occasionally and thermostating the entire instrument; no moving 
parts were involved. 



In this talk I will give a brief review of the evolution of the 
instruments known as IRIS (InfraRed Interferometric Spectrometer). I will 
then discuss the concept of remote sensing by infrared emission spectroscopy, 
and show examples of results obtained with IRIS. Finally, I will discuss the 
most recent results from Voyager with emphasis on those from Titan, showing 
that the ability to record a wide spectral range with good spectral and 
spatial resolution and high radiometric accuracy has contributed substantially 
to our new understanding of this most interesting companion of Saturn. 

The infrared interferometric spectrometer (IRIS) 

The first Nimbus interferometer was patterned after a breadboard which 

was constructed by L. Chaney from the University of Michigan and our group at 

111 
the Goddard Space Flight Center. After a successful balloon flight and 

extensive laboratory testing, the Michigan team pursued further balloon 

activities and the GSFC team space application. The conceptual layout of the 

interferometer is indicated in Fig. 2. Texas Instruments, Inc. in Dallas, 

Texas, built all of the apace flight versions of the instrument. 

The first launch in 1968 was a disaster due to a malfunction in the 
guidance system of the rocket. Months later, the U.S. Navy found badly 
corroded remnants of the spacecraft in the Pacific Ocean. A year later, 
Nimbus 3 was launched with a spare model of IRIS on board and achieved the 
desired polar orbit. The interferometer functioned well and the mission was a 
success. A conventional grating spectrometer, SIRS, ' and IRIS • obtained 
vertical temperature profiles of the atmosphere; in addition, IRIS obtained 
water vapor and ozone distributions. Today similar measurements are carried 
out routinely by operational weather satellites. 

On Nimbus ^ the resolved spectral interval was decreased from 5 to 2.8 

cm"" and several other design changes contributed to the generally better 

19 
performance of this instrument, as compared to its predecessor. After a 

year's operation, the instrument was turned off because we were inundated by 

data. 



A raajor design change was implemented in the next generation of IRIS, 

20 

earmarked to fly on Mariner 8 and 9 to Mars. In the Nimbus instrument, the 
potassium bromide beamsplitter material limited the spectral range to UOO 
cm" . However, the range between 200 and UOO cm" contains strong rotational 
water vapor lines, crucial to the Mars investigation. A change to cesium 
iodide was therefore made, although Csl is very soft, hard to polish and 
difficult to maintain flat. Again one of the two spacecraft. Mariner 8, was 
lost, this time in the Atlantic Ocean, but after a 6 month cruise Mariner 9 
reached Mars and achieved the desired orbit. IRIS and the spacecraft 

performed beyond expectation, for eleven months, until the supply of attitude 

7 8 9 
control gas was depleted.'' '-^ 

The largest and most ambitious step in the evolution of IRIS came in 
response to demanding requirements for exploration of the outer planets. 
Temperatures there are only slightly higher than that of liquid nitrogen; 
under these circumstances the measurement of the thermal emission spectrum is 
not an easy task. Moreover, the instruments had to survive for years in space 
and had to function in the severe high-energy particle environment which 
exists in the vicinity of Jupiter. 

21 
The optical layout of the Voyager IRIS is shown in Fig. 3. The whole 

instrument, including the half-meter telescope, weighs only 18.M kg and 

operates with an average power of 14 Watt. The Cassegrain telescope forms an 

image of the object at the focal plane aperture which limits the field of view 

to 0.25 full cone angle. A dichroic mirror channels the visible and near 

infrared portion of the spectrum into a radiometer, and the lower wavenumbers 

into the Michelson interferometer which analyzes the spectrum between 180 and 

2500 cm" with a 4.3 cm" apodized resolution. The main interferometer and 

the reference interferometer, which controls the motor speed and the 

wavenumber calibration, are shown for convenience in Fig. 3 in the plane of 

the paper, although they are in reality perpendicular to it. As all previous 

IRIS, the Voyager instrument is therraostated by thermally insulating the 

entire aj^sembly from the spacecraft, and allowing the instrument to cool by 

radiating to space. The instrument is held at a constant temperature of 200 K 

by the thermostatic action of small electrical heaters. Voyager IRIS has 



three independent thermostats, one for the interfsrometer proper, one for the 
primary, and one for the secondary telescope mirror. The Voyager 
interferometers have performed well^^ although a slight optical misalignment, 
more pronounced on Voyager 2 than on 1, has been noted. 

Remote sensing concept and results 

The art of remote sensing is to infer physical and chemical conditions 
from radiance measurements at different wavenumbers and zenith angles. This 
task must be based on radiative transfer theory. The physical quantity 

measured by these interferometers is the spectral radiance expressed, for 

—2 —1 —1 1 
example, in W cm sr (om ) . Sometimes it is more instructive, as in the 

case of Fig. 1, to plot the spectra in units of brightness temperature, 

defined as the temperature of a blackbody which emits, at a particular 

wavenumber, an equal amount of radiation as the object under investigation. 

Restricting the case to thermal emission from a plane parallel atmosphere 
in thermodynamic equilibrium, of optical thickness t^, above a lower boundary 

of emissivity Cp and temperature Tp, the spectral radiance can be expressed 

.22 ^ 

by 

-ll , _1 

Ko.y) = c^BCTq) e »» + 1 / 1 BCT(T)]e •'dr. (1) 

o 

B is the Planck function and cos" m the emission angle. All quantities in Eq. 
1, except V and T, depend on the wavenumber, v. The optical depth t is 
defined by 



tCv.z) = / e Ck^(v,z',T) p^<z',T)] dz', (2) 

z i 

where k^ and p^ are the absorption coefficient and density of gas i, and z and 
z' are altitudes. Eqs. \ and 2 are valid only for monochromatic radiation; a 
convolution with the instrument function is required before computed and 
observed radiances can be compared. 



The first term in Eq. 1 represents emission from a solid surface and the 
second term emission from atmospheric layers. The Galilean satellites, except 
lo, and the satellites of Saturn, except Titan, have virtually no atmosphere; 
therefore only the first term in Eq. 1 needs to be considered. The measured 
infrared spectra from these airless bodies follow reasonably closely the 
energy distribution of a blackbody; this allows a precise temperature 
measurement but says little about the chemical composition of the surface. 
Even Europa, which from ground -t^vsad near-infrared measurements -* is known to 
have water ice on its surface, did net shv^y the signatures of ice in ♦ihe far 

infrared. In contrast to this, small crystals of water ice suspended in the 

25 
atmosphere of Mars showed strong characteristic signatures of ice, as shown 

in Fig. 4. A similar phenomenon was observed with the fine dust suspended in 



the atmosphere of Mars. The dust displayed prominent spectral features 
shortly after arrival of Mariner 9 during the great dust storm of 1971, but 
the features diminished with clearing and settling of the dust. While 
suspended, dust absorption and emission affected the spectrum by contrast 
against the warmer or cooler background, but while on the surface the 
temperature contrast was small and the spectral features had almost 
disappeared. 

The only case where we have observed a surface emissivity effect with 

2 "^ 
certainty is in the desert areas on Earth. •"' Even in the presence of an 

atmosphere, the surface may be observed in spectral regions where t^ is small 

compared to unity, as shown in Fig. 5. The variation of the surface 

emissivity with wavenumber gives a clue to the chemical composition. SiOo in 

coarse quartz sand shows strong reststrahlen features in the spectrum of the 

Sahara. Global maps of this feature, for example, illustrate the distribution 

of deserts on Earth. 



The thermal emission from lo is also predominantly from the surface. 

Only near 1350 cm" have SO- gas and possibly SOp ice crystals been detected 

1 1 
by IRIS in a region containing a volcanic plume, as shown in Fig. 6. Many 

lo spectra show surface emission from hot spots of higher than ambient 

10 24 
temperatures indicating volcanic activity on a large scale. ' Some of the 

low wavenumber features of lo have escaped identification so far. 



We believe the solid (or liquid) surface of Titan is observed at 
approximately 550 cm" , with only a small atmospheric opacity due to the wings 

of pressure-induced nitrogen ar.d hirdrogen lines and residual absorption by 

1^ 27 
methane clouds. -" ' 



Now we shall turn our attention to atmospheric emission, that is to the 
second term of Eq. 1, which dominates in spectral regions where t. is large. 
The task of deriving atmospheric temperatures from a measurement of I 

V 

requires an inversion of the integral equation 1 which, in the early days of 
remote sensing, was often compared to the task of reconstructifig an egg from 



its scrambled state. However, much has been learned about this process in the 
last decade so that today the inversion technique is generally not a 
limiting factor in the interpretation of planetary spectra. 



One can always compute a synthesized spectrum by assuming a vertical 
profile of temperature and a reasonable distribution of atraospiieric 
constituents, comparing the calculated to the observed spectrum, making 
adjustments to the assumptions, and iterating the process until agreement 
exists across the spectrum. A careful error analysis must be performed 
because solutions are not always unique. All inversion procedures assume that 
the absorption coefficients of all absorbers involved are adequately known as 

functions of temperature and pressure. Line by line computational methods and 

2Q 
molecular parameters are now available for many molecules in the form of 

listings of line positions and strength on magnetic tape.-* However, for many 

other molecules such listings are often incomplete or nonexistent. Some of 

the more complex hydrocarbons, which we found on Titan fall into the latter 

category; it makes interpretation of spectra and determination of abundances 

often difficult.^ ''-^ Even for COp, which has been studied extensively 

because of its importance for the retrieval of vertical temperature profiles 

in the Earth's atmosphere, it was necessary to include many very weak bands 

1 '3 17 

and even bands of isotopes, including -^C and '0 into our molecular models 
before the spectrum of the 667 cm" Martian COp band, shown in Fig. 7, was 
fully under stood. -^-^ 



Extraction of the temperature profile from a planetary emission spectrun 
requir'.>3 analysis of a spectral region where a single uniformly mixed gas of 
known aC'undance is the dominant emitter. In the terrestrial spectrum the high 
wavenumber side of the 667 cm" COp band (see Figs. 1 and 5) is well suited 
for that purpose and has been the main spectral interval for temperature 
sounding on an operational basis. Weak absorption by 0- and HgO lines, also 
present in thi^j region, can be accounted for in the analysis. As mentioned 

Q 

before, the same cGp band served our Mars investigation. On Jupiter and 
Saturn pressure-induced absorption lines of hydrogen, the major constituent in 
those atmospheres, permitted retrieval of temperatures between 100 and 700 mb. 

The strong CHh band at 1304 cra~ allowed extension of the profiles up to 

10 12 1^ 
altitudes corresponding to a pressure of about 1 mb.' • ' -^ 

On Titan matters are more complicated. Direct temperature retrieval in 
the CHn band is possible from the IRIS spectra between about 1 and 20 mb. For 
higher pressures no suitable spectral region was found for a direst tempera- 
ture inversion. Pressure-induced hydrogen lines are present, but are very 
weak, and exist in a spectral region where other absorbers, probably methane 
clouds, interfere. ' Fortunately, the Radio Science team on Voyager^ ob- 
tained a T/m profile of Titan's atmosphere CT is the temperature and ra the 
mean molecular weight) . Combining this profile with IRIS derived temperatures 
yielded the actual temperature profile from 1 to 1600 mb, that is from the 
high stratosphere to the surface, as well as a mean molecular weight of about 

28.6 AMU. The latter value suggests an No atmosphere with an admixture of a 

27 
heavier gas, possibly argon. Vapor pressure considerations limit the 

stratospheric CHn content to ,r2.7%; in the troposphere the CH,. content seems 

27 
to be higher, but only about 0.6 of the saturation level. This picture is 

consistent with a stratified cloud layer composed of CH^ ice crystals just 

below the tropopausa and a relatively clear zone below the cloud deck filled 

only with a slowly settling smog consisting mostly of solid hydrocarbons. 

Temperature profiles of Titan and other planets are summarized in Fig. 8. 

After having established the temperature profile, that is after having 
solved Eq. 1 for T(t) for a uniformly mixed known constituent, one may reverse 
the process and solve for the opacity distribuiton, t{T3 , of an unknown 

8 



atmospheric constituent by using a spectral region where emission from the 

1 ? 1 ! 

latter dominates. This has been done for water vapor and ozone on Earth '' 
and for NH^ on Jupiter. However, in many cases the weakness of spectral 
features of minor constituents haa only allowed establishment of their 
existence or mean abundances. The atmospheric composition of Titan is listed 
in Table 
9 and 10. 



27 32 
in Table 1. ''-^ Gases are also identified in the Titan spectra shown in Fig. 



In addition to gases which have specific features in the measured 
spectra, helium can be identified in an indirect way. Helium atoms colliding 
with hyarogtrii mC-'^ccules change the shape of the broad pressure induced lines 

of hydrogen.^ Analysis of the line shape of the H, features in the 200-600 

—1 
cm range has permitted the derivation of the helium abundances on Jupiter 

and Saturn. The mass fraction of helium on Jupiter was found to be 0.19 ± 

0.05 from the IRIS spectra alone and 0.21 ± 0.06 from a technique which 

combines IRIS and Radio Science results,-^ For Saturn the IRIS spectra 

yielded a lower helium abundance of only 0.11 with an error not yet precisely 

1 "^ 
determined, but probably smaller than on Jupiter. "^ Theories of the interior 

structure of the giant planets have predicted this depletion. ^°'^^''' 

According to these theories the atmosphere of Saturn should be differentiated 

by gravitational forces, causing depletion of helium in the outer layers and 

enrichment in the interior. The sinking of helium liberates gravitational 

energy which is converted to heat and contributes to the excess of primordial 

heat emitted by both giant planets. On Jupiter, this excess of thermal 

radiation has been determined from IRIS data to be 1.67 ± 0.09 times the 

11 
energy Jupiter receives from the Sun. The IRIS analysis of the excess 

energy of Saturn has not yet been completed, but preliminary estimates and 

42 
previous measurements by Pioneer suggest that Saturn's excess energy 

fraction will be at least as large as Jupiter's. 



The discussion of interesting results obtained by IRIS is far from 
complete. Time does not permit me to show the wind field derived from 
temperature data on Mars,"'^ or the wind shear computations on Jupiter, '^^ 

11^ 111! he 

Saturn -" and Titan, or the dynamics of the Great Red Spot.^ I will not 
discuss IRIS results on the rings of Saturn^^ or the topography of Mars.^ In 
spite of these and other omissions, I hope I have given you an overview of 



some of the highlights and shown to you the merits of Fourier transform 
spectroscopy in the thermal infrared. 

I would like to mention that the IRIS data for the Voyager Jupiter 
encounter and the earlier missions are stored on magnetic tapes. Resfcarehers 
may request copies from the Space Science Data Center at Goddard Space Flight 
Center, Greenbelt, MD 20771. Saturn data will become available to the 
scientific community in early 1982. I thank B. Corrath and J. Pearl for 
critically reading the manuscript. 



10 



Table 1. Atmospheric Composition of Titan 
Inferred from Voyager IRIS^^'^' "'-^^ 







Wave 


Approximate 


Chemical 




Number 


Mole 


Family 


Gas 


(cm" ) 


Fraction 


Major Constituents 










Nitrogen, Np 


« 


^ 0.85 




Argon, A 


* 


^ 0.12 




Hydrogen, H 


350 


2 X 10"^ 


Carbon-Hydrogen 










Methane, CH^ 


1304 


^3 X 10"^ *« 




Ethane, CgHg 


822 


2 X 10""^ 




Propane, C-Hg 


748 


1 X 10'^ 
5 X 10-^ 




Acetylene, GgHp 


729 




Ethylene, CJti^ 


950 


8 X 10'*^ 
6 X 10'^ 




Methyl Acetylene, C^H^ 


325,633 




Diacetylene, C^Hg 


220,628 




Carbon-Hydrogen-Nitrogen 









Hydrogen Cyanide, HCN 712 5 x 10 
Cyanoacetylene, HC-N 500,663 



-7 



Carbon-Nitrogen 



Cyanogen, CgNg 



233 



* Determined from mean raoleculan weight obtained in conjunction with Voyager 

Radio Science Investigation. ^^"^''^^ 
»* Variable with altitude, less than 2.7% in the stratosphere and possibly as 

high as 8% near the surface. 



11 



References 

1. Hanel, R. and Conrath, B. "Interferometer Experiment on Nimbus 3: 
Preliminary Results", Science , Vol. 165, pp. 1258-1260, 1969. 

2. Conrath, B. J., H^^nel, R. A., Kunde, V. G. and Prabhakara, C, "The 
Infrared Interferometer Experiment on Nimbus 3", J. Geophys. Res ., Vol. 75, 
Ko. 30, pp. 583^-5856, 1970. 

3. Hanel, R. A., Conrath, B. J., Kunde, V. G., Prabhakara, C, Revah, I., 
Saloraonson, V. V. and Wulford, G., "The NiMbus H Infrared Spectroscopy 
Experiment: 1. Calibrated Thermal Emission Spectra", J. Geophys. Res . , Vol. 
77. PP. 2639-2641, 1972. 

4. Kunde, V. G. , Conrath, B. J., Hanel, R. A., Haguire, W. C, Prabhakara, 



C. and Saloraonson, V. V., "The Nimbus M Infrared Spectroscopy Experiment: 2. 
Comparison of Observed and Theoretical Radiances i 
Geophys. Reo .. Vol. 79, No. 6, pp. 777-781, 1974. 



Comparison of Observed and Theoretical Radiances from 425-1450 cm" ", £^ 



5. Prabhakara, C, Rodgers, E. B. , Conrath, B. J., Hanel, R. A., Kunde, V. 
G., "The Nimbus 4 Infrared Spectroscopy Experiment: 3. Observations of the 
Lower Stratospheric Thermal Structure and Total Ozone", J. Geophys. Res . , Vol. 
81, No. 36, pp. 6391-6399, 1976. 

6. Kempe, V., Oertel, D. , Puder, J., Roseler, A., Sakatov, D. P. and 
Studemund, H., "Infrarot Fourier Spectrometer Sl-1 auf Meteor-25" , Radio 
Fernsehen Electronik . Vol. 26, pp. 627-630, 1977. 

7. Hanel, R. h,^ Conrath, B. J., Hovis, W. A., Kunde, V. G., Lowman, P. P., 
Pearl, J. C. , Prabhakara, C, Schlachraan, B. , "Infrared Spectroscopy 
Experiment on the Mariner 9 Mission: Preliminary Results", Science , Vol. 175, 
pp. 305-308, 1972. 

8. Hanel, R., Conrath, B. , Hovis, W. , Kunde, V., Lovanan, P., Maguire, W., 
Pearl, J., Pirraglia, J., Prabhakara, C, Schlachman, B., Levin, G., Straat, 
P., and Burke, T. , "Investigation of the Martian Environment by Infrared 
Spectroscopy on Mariner 9", Icarus , Vol. 17, pp. 423-442, 1972. 

12 



9. Conrath, B., Curran, R., Hanel, R., Kunde, V., Maguire, W., Pearl, J., 
Pirraglia, J., Welker, J., and Burke, T., "Atmospheric and Surface Properties 
of Mars Obtained by Infrared Spectroscopy on Mariner 9", J. Geophys. Res . , 
Vol. 78, No. 2i>, pp. »<267-'*278, 1973. 

10. Hanel, R., Conrath, B. , Flasar, H., Kunde, V., Lowman, P., Maguire, W., 
Pearl, J., Pirraglia, J., Samuelson, R., Gautier, D., Gierasch, P., Kumar, S., 
and Ponnamperuraa , C, "Infrared Observations of the Jovian System from Voyager 
1", S cience . Vol. ^0^, pp. 972-976, 1979. 

11. Pearl, J., Hanel, R., Kunde, V., Maguire, W., Fox, K., Gupta, S., 
Ponnamperuraa, Cc, and Raulin, F., "Identification of Gaseous SOg and New Upper 
Liraitd for Other Gases on lo" , Nature , Vol. 280, No. 5725, pp. 755-758, 1979. 

12. Hanel, R., Conrath, B., Flasar, M., Herath, L., Kunde, V., Lowraan, P., 
Maguire, W. , Pearl, J., Pirraglia, J., Samuelson, R., "Infrared Observations 
of the Jovian System from Voyager 2", Science, Vol. 206, No. 4421, pp. 
952-956, 1979. 

13. Hanel, R. , Conrath, B. , Flasar, F. M., Kunde, V., Maguire, W., Pearl, J., 
Pirraglia, J., Samuelson, R., Herath, L., Allison, M., Cruikshank, D. , 
Gautier, D. , Gierasch, P., Horn, L., Koppany, R,, Ponnamperuraa, C, "Infrared 
Observations of the Saturnian System from Voyager 1", Science , Vol. 212, No. 
4491, pp. 192-200, 1981. 

14. Chaney, L. W., Drayson, S. R., and Young, C, "Fourier Transform 
Spectrometer-Radiative Measurement and Temperature Inversion", Appl . Opt . , 
Vcl. 6. pp. 347-349. 1967. 

15. Hanel, R. A., and Chaney, L. W. , "The Merits and Shortcomings of a 
Michelson Type Interferometer to Obtain the Vertical Temperature and Humidity 
Profile", Proc. XVII International Astronautical Congr ., Madrid, Vol. 2, 1966. 

16. Hanel, R., Schlachman, B., Clark, F. D., Prokesh, C. H., Taylor, J. B., 
Wilson, W. M., and Chaney, L., "The Nimbus III Michelson Interferometer", 
Appl. Opt .. Vol. 9, No. 8, pp. 1767-1774, 1970. 

13 



17. Wark, D. Q. , and Hilleary, D. T., "Atmospheric Temperature: Successful 
Test of Remote Probing", Science , Vol. 165, pp. 1J?56-1258, 1969. 

18. Hanel, R. A., and Conrath, B. J., "Thermal Emission Spectra of the Earth 
and Atmosphere Obtained from the Nimbus H Miehelson Interferometer 
Experiment", Nature, Vol. 228, pp. I^S-TIS, 1970. 

19. Hanel, R. A., Schlachman, B. , Rodgers, D., Vanous, D., "The Nimbus ^ 
Miehelson Interferometer", Appl. Opt .. Vol. 10, pp. 1376-1381, 1971. 

20. Hanel, R. A., Schlachman, B., Breihan, E., Bywaters, R., Chapman, F., 
Rhodes, M., Rodgers, D., and Vanous, D. , "Mariner 9 Miehelson Interferometer", 
Appl. Opt. , Vol. 11, pp. 2625-263^, 1972. 

21. Hanel, R. , Crosby, D., Herath, L. , Vanous, D. , Collins, D. , Creswick, H. , 
Harris, C, and Rhodes, M., "Infrared Spectrometer for Voyager", Appl . Opt . , 
Vol. 19, No. 9, pp. 1391-1'<00, 1980. 

22. Hanel, R. , Conrath, B., Gautier, D. , Gierasch, P., Kumar, S. , Kunde, Vc, 
Lowraan, P., Maguire, W., Pearl, J., Pirraglia, J., Ponnamperuma , C, and 
Samuelson, R., "The Voyager Infrared Spectroscopy and Radiometry 
Investigation", Space Science Reviews , Vol. 21, pp. 129-157, 1977. 

23. Pilcher, C. B., Ridgway, S. T. and McCord , T. B. , "Galilean Satellites: 
Identification of Water Frost", Science , Vol. 178, pp. 1087-1089, 1972. 

24. Pearl, J., and Sinton, W., "Hot Spots of lo" , in The Satellites of 
Jupiter , D. Morrison, Ed., U. of Arizona Press, Tucson, AZ, 1981. 

25. Curran, R. , Conrath, B., Hanel, R., Kunde, V., Pearl, J., "Mars: Mariner 

9 Spectroscopic Evidence for HpO Ice Clouds", Science , Vol. 182, 381-383, 1973. 

26. Prabhakara, C, and Dalu, G., "Remote Sensing of the Surface Emissivity 
at 9 ym over the Globe", J. Geophys. Res ., Vol. 81, pp. 3719-3721, 1975, 



14 



27. Samuelson, R. E. , Hanel, R. A., Kunde, V. G. , and Maguire, W. C, "The 
Mean Molecular Weight and Hydrogen Abundance of Titan's Atmosphere", subraitted 
to Nature . 1981. 

28. Deepak, A., ed. "Inversion Methods In Atmospheric Remote Sounding ", 
Academic Press, 1977. 

29. Kunde, V. G. , and Maguire, W. C, "Direct Integration Transmittance 
Model", J. Quan^.. Spectrosc. Radiat. Transfer , Vol. ^^^, pp. 803-8l7t 1974. 

30. Rothman, L. S., Appl. Opt ., Vol. 17, 3517-3518, 1978. 

31. Maguire, W. C, Hanel, R. A., Jennnings, D. E., Kunde, V. G., and 
Samuelson, R. E. , "Propane and Methyl Acetylene in Titan's Atmosphere", 
subraitted to Nature , 1981. 

32. Kunde, V. G., Aikin, A. C, Hanel, R. A., Jennings, D, E.* Maguire, W. 
C, and Samuelson, R. E., "Identification of C^H-, HC-N and C-N- in the 
Atmosphere of Titan", submitted to Nature , 1981. 

33. Maguire, W. C, "Martian Isotopic Ratios and Upper Limits for Possible 
Minor Constituents as Derived from Mariner 9 Infrared Spectrometer Data", 
Icarus , Vol. 32, pp. 85-97, 1977. 

3'*. Tyler, G. L. , Eshleman, V. R., Anderson, J. D. , Levy, G. S. , Lindal, G. 
F., Wood, G. E., and Croft, T. A., "Radio Science Investigations of the Saturn 
System with Voyager 1: Preliminary Results", Scienc e , Vol. 212, No. 4491, pp. 
201-105, 1981. 

35. Kunde, V., Hanel, R. , Herath, L. , Maguire, W., Gautier, D. , Baluteau, J. 
P., Marten, A., Chedin, A., Husson, N., and Scott, N. , "The Lower Atmospheric 
Composition of Jupiter's North Equatorial Belt from the Voyager 
Investigation", in preparation. 

35. Birnbaum, G. , "Far Infrared Absorption in Hg-Hg and Hg-Ht Mixtures", J_^ 
Q uant. Spectrosc. Radiat. Transfer , Vol. 19, pp. 51-62, 19'|8. 

15 



37. Gautier, D., Conrath, B., Flasar, M., Hanel, R., Kunde, V., Chedin, A,, 
and Scott, N., "The Helium Abundance of Jupiter from Voyager", J. Geophys. 
Res . , accepted for publication, 1981. 

38. Salpeter, E. E. , "On Convection and Gravitational Layering in Jupiter and 
in Stars of Low Mass", Astrophys. J ., Vol. 181, pp. L89-92, 1973. 

39. Stevenson, D. J., and Salpeter, E. E., "The Phase Diagram and Transport 
Porperties for Hydrogen-Helium Fluid Planets", Astrophys. J. Suppl ., Vol. 35, 
pp. 221-237, 1977. 

40. Pollack, J. B. , Grossman, A. S. , Moore, R., and Graboske, H. C, "A 
Calculation of Saturn's Gravitational Contraction History", Icarus , Vol. 30, 
pp. 111-128, 1977. 

nia iialicJLi 11 • n»f lA/tit auUy Um u«y 11^=1 awiiy Li* "•# t\UtiUi;fy *« ^^t ailU ixita^^xxay 

J. A., "Albedo, Internal Heat, and Energy Balance of Jupiter: Preliminary 
Results of the Voyager Infrared Investigation", J. Geophys. Res ., accepted for 
publication, 1981. 

42. Ingersoll, A. P., Orton, G. S. , Munch, G. , Neugebauer, G., Chase, S. C, 
"Pioneer Saturn Infrared Radiometer: Preliminary Results", Science , Vol. 207, 
No. 25, pp. 439-443, 1980. 

43. Pirraglia, J. A., Conrath, B. J., Allison, M. D. , Gierasch, P. J., 
"Global Thermal Structure and Dynamics of Saturn and Jupiter from Voyager 
Infrared Measurements", submitted to Nature , 1981. 

44. Flasar, F. M., Samuelson, R. E., and Conrath, B. J., "Global Temperature 
Distribution and Dynamics of Titan's Atmosphere", submitted to Nature , 1981. 

45. Flasar, F. M., Conrath, B. J., Pirraglia, J. A., Clark, P. C, French, R. 
G., and Gierasch, P. J., "Thermal Structure and Dynamics of the Jovian 
Atmosphere, I. The Great Red Spot", J. Geophys. Res ., accepted for 
publication, 1981. 



16 



Figure Captions 

Fig. 1. Planetary emission spectra. The uppermost spectrum was recorded by 

Nimbus 1 in 1970 over the mid-Atlantic Ocean. The second spectrum is 
of mid-latitudes of Mars recorded in 1972 by Mariner 9. The spectra 
of Jupiter, Saturn and Titan have been taken by Voyager 1 in 1979 and 
1980 respectively. 

Fig. 2. Conceptional diagram of IRIS. For Nimbus and Mariner the image 

motion conpensation and calibration mirror can be oriented so that 

15 10 20 
IRIS sees the planet, deep space or an onboard blackbody. 

Fig. 3. Optical layout of the Voyager infrared instrument. Calibration of 
the interferometer is accomplished by occasionally observing deep 
space and by precise temperature control of interferometer and 
telescope. Calibration of the radiometer is accomplished by 
occasionally viewing a diffusor plate mounted on the spacecraft and 
illuminated by the Sun. 

Fig. 4. Mariner 9 spectra of Tharsis Ridge and Arcadia. Simultaneously taken 
images show partial cloudiness over Tharsis Ridge and little 

cloudiness over Arcadia. Calculated ice cloud spectrun in lower 

25 
panel is for comparison. 

Fig. 5. Thermal emission spectra of Mars and Earth, a. and b. Martian south 
polar spectra after and during the dust storm, c. Martian 
mid-latitude spectrum with strong features due to silicate dust. d. 
Reststrahlen spectrum of Quartz sand. e. Sahara spectrum showing 
the reststrahlen feature in the atmospheric window between 1050 and 
1250 cm'"*.^ 

Fig. 6. SOg gas on lo. Comparison of measured spectrum of lo with 
synthesized spectra in the vicinity of the v_ band of SO . 



17 



Fig. 7. Comparison of measured and synthesized spectra of Mars in the region 
of the strong 667 cm" COg band. The main spectral features are 
labeled with a three-digit number representing the isotope; thus 
16q13^18q j^g abbreviated 638. Unlabeled features are due to the main 
isotope O^^C 0. The synthetic spectrum is displaced 20.3 K. 

Fig. 8. Atmospheric temperature profiles of Earth, Mars, Jupiter, Saturn, and 
Titan derived from infrared spectra and in the case of Titan in 
combination with Radio Occultation data. The profiles of Earth and 
Mars are typical of low latitudes; polar profiles are much colder and 
different in shape. Variations with latitude are generally smaller 
on the outer planets and on Titan. 

Fig. 9. Average of 3 spectra of thP Titan's north limb and two laboratory 

33 
spectra of cyanoacetylene (HC-N) and cyanogen (C^Np). Other 

spectral features are labeled. 
Fig. 10. Disk and north polar spectra of Titan and laboratory spectrum of 

■3-3 

diacetylene (Cj^Hg) . 



18 



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